Systems for modifying pressure differential in a chemical vapor process

ABSTRACT

A process for densifying an annular porous structure comprising flowing a reactant gas into an inner diameter (ID) volume and through an ID surface of the annular porous structure, flowing the reactant gas through an outer diameter (OD) surface of the annular porous structure and into an OD volume, flowing the reactant gas from the OD volume through the OD surface of the annular porous structure, and flowing the reactant gas through an ID surface of the annular porous structure and into the ID volume.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of, claims priority to and the benefitof, U.S. Ser. No. 15/941,570 filed Mar. 30, 2018 and entitled “METHODSFOR MODIFYING PRESSURE DIFFERENTIAL IN A CHEMICAL VAPOR PROCESS,” whichclaims priority to and benefit of, U.S. Ser. No. 15/056,358 filed Feb.29, 2016 and entitled “METHODS FOR MODIFYING PRESSURE DIFFERENTIAL IN ACHEMICAL VAPOR PROCESS,” which is hereby incorporated herein in itsentirety for all purposes. Both of the aforementioned are herebyincorporated by reference.

BACKGROUND

Chemical vapor infiltration and deposition (CVI/CVD) is a known processfor making composite structures such as carbon/carbon brake disks. TheCVI/CVD process typically used for making carbon/carbon brake disks issometimes referred to as “conventional” or “isothermal” CVI/CVD. Thisprocess involves passing a reactant gas or gas mixture (e.g., methane,propane, etc.) around heated stack of porous structures (e.g.,carbonized stack of porous materials) at absolute pressures as low as afew torr (˜400 Pa or less). The gas diffuses into the stack of porousmaterials, driven by concentration gradients, and undergoes a CVDreaction such as thermal decomposition, hydrogen reduction,co-reduction, oxidation, carbidization, or nitridation to deposit abinding matrix.

During CVI/CVD, pores on the exterior of a stack of porous structuresmay become occluded. To resolve, one may remove the stack of porousstructures from the process vessel and machine the stack of porousstructures to open the pores. CVI/CVD may then be resumed.

SUMMARY

In various embodiments, a process is disclosed for densifying an annularporous structure comprising flowing a reactant gas into an innerdiameter (ID) volume and through an ID surface of the annular porousstructure, flowing the reactant gas through an outer diameter (OD)surface of the annular porous structure and into an OD volume, flowingthe reactant gas from the OD volume through the OD surface of theannular porous structure, and flowing the reactant gas through an IDsurface of the annular porous structure and into the ID volume.

In various embodiments, the process further comprises flowing thereactant gas into a preheater, wherein the preheater heats the reactantgas to a defined temperature before flowing into the ID volume. Invarious embodiments, the ID volume is defined by the annular porousstructure and a graphite susceptor. In various embodiments, the reactantgas comprises at least one of methane, ethane, propane, cyclopentane,hydrogen, nitrogen, helium, argon, or an alkane. In various embodiments,the graphite susceptor is disposed within a furnace. In variousembodiments, the annular porous structure comprises a carbon fiber. Invarious embodiments, the flowing the reactant gas into the ID volumecause an increase in pressure of the ID volume. In various embodiments,the increase in pressure of the ID volume creates a pressuredifferential between the ID volume and the OD volume. In variousembodiments, the pressure differential drives the reactant gas toinfiltrate a pore of the annular porous structure.

In various embodiments, the process further comprises adjusting a plugdisposed in a lid of a graphite susceptor, the graphite susceptorsupporting the annular porous structure. In various embodiments, theprocess further comprises energizing a reversing valve and unenergizingthe reversing valve. In various embodiments, the unenergizing thereversing valve occurs in response to a command from a processor.

In various embodiments, the present disclosure provides an apparatus fordensifying an annular porous structure comprising a furnace, a graphitesusceptor disposed in the furnace, and a reversing valve, the reversingvalve having a first port and a second port, wherein the first port iscoupled to a first inlet/outlet in the furnace via a first line, andwherein the second port is coupled to a second inlet/outlet in thefurnace via a second line. In various embodiments, the reversing valvehas an energized state and an unenergized state. In various embodiments,in the energized state, the first port conveys a reactant gas to thefurnace and into an inner diameter (ID) volume. In various embodiments,in the energized state, the second port exhausts the reactant gasthrough at least one of a second opening disposed in the graphitesusceptor and a third opening disposed in the graphite susceptor. Invarious embodiments, in the unenergized state, the second port conveysthe reactant gas to the furnace and into an outer diameter (OD) volume.In various embodiments, in the unenergized state, the first portexhausts reactant gas through a first opening disposed in the graphitesusceptor.

In various embodiments, the furnace is at least one of gas heated orinduction heated. In various embodiments, the graphite susceptorsupports the annular porous structure. In various embodiments, thesecond opening is in communication with the OD volume and the thirdopening is in communication with the OD volume. In various embodiments,the first opening in communication with the ID volume. In variousembodiments, the apparatus further comprises a spacer disposed incontact with the annular porous structure. In various embodiments, theapparatus further comprises a vacuum pump in fluid communication withthe reversing valve. In various embodiments, the apparatus furthercomprises a reactant gas source in fluid communication with thereversing valve. In various embodiments, the apparatus further comprisesa processor configured to toggle the reversing valve from the energizedstate to the unenergized state.

In various embodiments, the present disclosure provides an apparatus fordensifying an annular porous structure comprising a graphite susceptorsupporting the annular porous structure, wherein the annular porousstructure defines an inner diameter (ID) volume and an outer diametervolume (OD), and wherein the graphite susceptor comprises a lid. Invarious embodiments, the apparatus further comprises a first plugdisposed in a first opening of the lid, the first opening in fluidcommunication with the ID volume. In various embodiments, the apparatusfurther comprises a second plug disposed in a second opening of the lid,the second opening in fluid communication with the OD volume. In variousembodiments, the apparatus further comprises a third plug disposed in athird opening of the lid, the third opening in fluid communication withthe OD volume. In various embodiments, the apparatus further comprises acontrol arm configured to remove at least one of the first plug, thesecond plug, or the third plug.

In various embodiments, the graphite susceptor is disposed in a furnace.In various embodiments, the furnace is at least one of gas heated orinduction heated. In various embodiments, the control arm is configuredto insert the first plug into the first opening, the second plug intothe second opening, and the third plug into the third opening. Invarious embodiments, the first plug, the second plug, and the third plugcomprise graphite. In various embodiments, the control arm is controlledby a mechanical controller.

In various embodiments, the apparatus further comprises a spacerdisposed in contact with the annular porous structure. In variousembodiments, the apparatus further comprises a vacuum pump in fluidcommunication with the furnace. In various embodiments, the apparatusfurther comprises a reactant gas source in fluid communication withfurnace, wherein the reactant gas comprises at least one of methane,ethane, propane, cyclopentane, hydrogen, nitrogen, helium, argon, or analkane. In various embodiments, the annular porous structure comprisescarbon fiber.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a schematic view of a chemical vapor infiltrationfurnace configured for inner diameter (ID) gas feed, in accordance withvarious embodiments;

FIG. 2 illustrates a schematic view of a chemical vapor infiltrationfurnace configured for outer diameter (OD) gas feed in accordance withvarious embodiments;

FIG. 3 illustrates a schematic view of a graphite susceptor configuredfor inner diameter (ID) gas feed using reversing valves in accordancewith various embodiments;

FIG. 4 illustrates a schematic view of a graphite susceptor configuredfor outer diameter (OD) gas feed using reversing valves in accordancewith various embodiments;

FIG. 5 illustrates a CVI/CVD process in accordance with variousembodiments; and

FIG. 6 illustrates a CVI/CVD process in accordance with variousembodiments.

DETAILED DESCRIPTION

All ranges and ratio limits disclosed herein may be combined. It is tobe understood that unless specifically stated otherwise, references to“a,” “an,” and/or “the” may include one or more than one and thatreference to an item in the singular may also include the item in theplural.

The detailed description of exemplary embodiments herein makes referenceto the accompanying drawings, which show exemplary embodiments by way ofillustration and its best mode, and not of limitation. While theseexemplary embodiments are described in sufficient detail to enable thoseskilled in the art to practice the disclosure, it should be understoodthat other embodiments may be realized and that logical, chemical andmechanical changes may be made without departing from the spirit andscope of the disclosure. For example, the steps recited in any of themethod or process descriptions may be executed in any order and are notnecessarily limited to the order presented. Furthermore, any referenceto singular includes plural embodiments, and any reference to more thanone component or step may include a singular embodiment or step. Also,any reference to attached, fixed, connected or the like may includepermanent, removable, temporary, partial, full and/or any other possibleattachment option. Additionally, any reference to without contact (orsimilar phrases) may also include reduced contact or minimal contact.

Carbon/carbon parts (“C/C”) in the form of friction disks (also referredto as a carbon/carbon brake disks) are commonly used for aircraft brakedisks, race car brakes, and clutch disks. Carbon/carbon brake disks areespecially useful in these applications because of the superior hightemperature characteristics of C/C material. In particular, thecarbon/carbon material used in C/C parts is a good conductor of heat andis able to dissipate heat generated during braking away from the brakingsurfaces. Carbon/carbon material is also highly resistant to heatdamage, and thus, is capable of sustaining friction between brakesurfaces during severe braking without a significant reduction in thefriction coefficient or mechanical failure. Furthermore, carbon/carbonbrake disks are useful because they are relatively light weight, inparticular in comparison to previous steel brakes.

One method of manufacturing C/C materials involves fabrication of apreform from an oxidized polyacrylonitrile (PAN) (also referred to as“OPF”) or carbon fiber, followed by carbonization and chemical vaporinfiltration (CVI) densification of the preform. As used herein, apreform may comprise any porous structure, and the terms preform,fibrous preform, and porous structure may be used interchangeably. TheCVI/CVD process cycles are continued, in conjunction with machining thepreform between infiltration cycles if desired, until the desired partdensity is achieved. In various embodiments, machining the surfaces ofthe preform may open surface porosity, thereby facilitating weightincreases (i.e., density increases) in the preform during subsequentdensification steps.

In general, C/C parts are produced using the OPF, carbonization, andCVI/CVD densification method are made in three successive manufacturingsteps. First, a fibrous preform is made utilizing a variety of textilemanufacturing techniques. Typically, the fibrous preform is made fromOPF or carbon fiber. Although numerous techniques are known in the artfor making fibrous preforms from OPF, a common technique involvesstacking layers of OPF to superimpose the layers. The added layers maythen be needled perpendicularly to the layers with barbed textileneedles. The needling process generates a series of z-fibers through thefibrous preform that extend perpendicularly to the fibrous layers. Thez-fibers are generated through the action of the needles pushing fibersfrom within the layer (x-y or in-plane) and reorienting them in thez-direction (through-thickness). Needling of the fibrous preform may bedone as one or more layers are added to the stack or may be done afterthe entire stack is formed. The needles may also penetrate through onlya portion of the preform or may penetrate through the entire preform. Inaddition, resins are sometimes added to the fibrous preform by eitherinjecting the resin into the preform following construction or coatingthe fibers or layers prior to forming the fibrous preform. Fibrouspreforms may also be made from pitch based carbon fiber tows and/or fromrayon carbon fiber tows.

After the fibrous preform is made, it is carbonized to convert the OPFinto carbon fibers in a process referred to herein ascarbonization/graphitization. Typically, fibrous preforms are carbonizedby placing the preforms in a furnace with an inert atmosphere. As iswell-understood by those in the art, the heat of the furnace causes achemical conversion which drives off the non-carbon chemicals from thepreform. Carbonization/graphitization may be conducted in a vacuum orpartial vacuum (e.g., at pressures of 1-15 torr) or in an inertatmosphere at a temperature in the range from about 1,400° C. to about2,800° C. (2,552° F. to about 5,072° F.), and in various embodiments inthe range from about 1,400° C. to about 2,500° C. (2,552° F. to about4,532° F.), and in various embodiments in the range from about 1,400° C.to about 2,200° C. (2,552° F. to about 3,992° F.)(wherein the term aboutin this context only means+/−100° C.) for a period of time in the rangeof up to about 60 hours, and in various embodiments, in the range up toabout 10 hours (wherein the term about in this context only means+/−2hours). The resulting preform generally has the same fibrous structureas the fibrous preform before carbonizing. However, the OPF have beenconverted to 100% carbon or very near 100%, for example from 95% carbonto 99.9% carbon. The resulting preform may be referred to as having afibrous network. In various embodiments, the preform may comprise anygeometry.

After the preform has been carbonized, the preform is densified. Thepreform may be referred to as a “stack of porous structures” before andduring densification. In general, densification involves filling thevoids, or pores, of the fibrous preform with additional carbon material.This may be done using the same furnace used for carbonization or adifferent furnace. Typically, chemical vapor infiltration and deposition(“CVI/CVD”) techniques are used to densify the fibrous preform with acarbon matrix. This commonly involves heating the furnace and thecarbonized preforms, and flowing a reactant gas comprising, for example,hydrocarbon gases (e.g., at least one of methane, ethane, propane,butane, and/or the like, as described herein) into the furnace andaround and through the fibrous preforms. The hydrocarbons may comprisealkanes, for example, straight chain, branched chain and/or cyclicalkanes, having from 1 to about 8 carbon atoms, and in variousembodiments from 1 to about 6 carbon atoms, and in various embodimentsfrom 1 to about 3 carbon atoms. Methane, ethane, propane, cyclopentane,or mixtures of two or more thereof may be used. The reactant gas maycomprise one or more alkanes of 2 to about 8 carbon atoms, and invarious embodiments from 2 to about 6 carbon atoms. Mixtures of one ormore alkanes of 1 to about 8 carbon atoms with one or more alkenes of 2to about 8 carbon atoms may be used. In various embodiments, the CVI/CVDprocess may include a temperature gradient. In various embodiments, theCVI/CVD process may include a pressure differential. As used herein,CVI/CVD may refer to chemical vapor infiltration or chemical vapordeposition. Accordingly, CVI/CVD may refer to chemical vaporinfiltration or deposition.

CVI/CVD densification may be conducted in a vacuum or partial vacuum(e.g., at pressures of 1-15 torr (133 Pa to 1999 Pa) or in an inertatmosphere at a temperature in the range from about 900° C. to about1100° C. (1,652° F. to about 2012° F.), and in various embodiments inthe range of up to about 1,000° C. (1,832° F.) (wherein the term aboutin this context only means+/−100° C.) for a period of time in the rangefrom about 150 hours to about 550 hours, and in various embodiments, inthe range from about 300 hours to about 700 hours (wherein the termabout in this context only means+/−24 hours). The number of hours usedin a CVI/CVD process may be referred to as hours on gas (HOG).

As a result, carbon decomposes or pyrolyzes from the hydrocarbonreactant gases and is deposited on and within the preforms. Typically,the densification process is continued until the preform reaches adensity in the range from 1.6 to 1.9 grams per cubic centimeter (g/cc),and in various embodiments, a density of approximately 1.75 g/cc. Whenthe densification step is completed, the resulting C/C part has a carbonfiber structure with a carbon matrix infiltrating the fiber structure,thereby deriving the name “carbon/carbon.”

The term “composite structure” may refer to a densified stack of porousstructures. The composite structure may comprise a stack of porousstructures with a solid residue or matrix dispersed throughout the stackof porous structures. The composite structure may comprise acarbonaceous stack of porous structures with a carbonaceous matrixdispersed in the stack of porous structures. This may be referred to asa carbon/carbon composite. The composite structure may comprise aceramic stack of porous structures with a ceramic or oxide matrixdispersed in the stack of porous structures. The composite structure maycomprise a mixed or hybrid composite structure such as a carbon stack ofporous structures with a ceramic or oxide matrix dispersed in the stackof porous structures, a carbon stack of porous structures with a mix ofcarbon and ceramic or oxide matrix dispersed in the stack of porousstructures, a ceramic stack of porous structures with a carbon matrixdispersed in the stack of porous structures, a ceramic stack of porousstructures with a mix of carbon and ceramic or oxide matrix dispersed inthe stack of porous structure, and the like. In various embodiments, thecomposite structure may comprise carbon, silicon, silicon carbide,silicon nitride, boron, boron carbide, aluminum nitride, titaniumnitride, cubic zirconia, and SiCxNy, where x is a number in the rangefrom about zero to about 1, and y is a number in the range from aboutzero to about 4/3.

The terms “higher order rough laminar structure,” “rough laminarmicrostructure,” “transitional microstructure,” “smooth laminarmicrostructure,” “transitional microstructure,” “dark laminar” and“isotropic” may be used to describe the microstructure of a compositestructure employing a carbon matrix dispersed in a stack of porousmaterials. The microstructure may be determined by use of polarizedlight microscopy. A carbon/carbon composite with a rough laminarstructure may be characterized as having high optical activity andnumerous irregular extinction crosses. A carbon/carbon composite with asmooth laminar structure may be characterized as having low opticalactivity and smooth extinction crosses. A carbon/carbon composite withlittle to no optical activity may be characterized as dark laminar orisotropic. These microstructures may be quantified in terms of theirextinction angles.

Composite structures made according to various embodiments may be usefulas carbon/carbon aircraft disk brakes, ceramic combustion and turbinecomponents such as turbine engine hot section components, ceramicfriction materials, ceramic heat sinks, and the like. The carbon/carbondisk brakes may be in the form of circular disks or disks.

As used herein, the term “stack of porous structures” may beinterchangeable with “porous structures stack.” A stack of porousstructures may comprise one or more sub-stack of porous structures thatare associated. For example, a stack of porous structures may comprisetwo sub-stack of porous structures coupled so that there is contactbetween each sub-stack of porous structures, such as in a “stack.” Astack of porous structures may comprise three or four sub-stack ofporous structures positioned so that at least two of the sub-stack ofporous structures are in contact with each other. For example, a stackof porous structures system may comprise four sub-stacks stack of porousstructures positioned in a stack formation.

The stack of porous structures may comprise a first surface, a secondsurface and at least one other surface connecting the first surface andthe second surface. In various embodiments, and as used herein, anysurface may be any shape such as, for example, at least one of rounded,sphere shaped, toroid shaped, or frustoconical.

In various embodiments, pressure differentials may also be used withthermal gradients. A pressure differential may be created when pressureon one surface of a stack of porous structures is different than thepressure at another surface of the stack of porous structures.

In conventional systems for CVI/CVD densification, soot and/or tar maycoat surfaces of the stack of porous structures. Soot may refer toundesirable accumulations of carbon particles on the furnace equipmentand/or stack of porous structures, and tar may refer to undesirableaccumulations of large hydrocarbon molecules on the furnaceequipment/stack of porous structures. The large hydrocarbon moleculesmay cause thick coatings on the surfaces of the stack of porousstructures. Typically, accumulations of soot and/or tar form when thereactant gas stagnates for a period of time in an area or comes intocontact with cooler furnace surfaces. Stagnation typically occurs inareas where the gas flow is blocked or where the gas flow is moving moreslowly than the surrounding gas flow.

Accumulations of soot and tar can cause a number of problems whichaffect both the quality of the composite structures and the costs ofmanufacturing. Seal-coating is one typical problem that can result fromsoot and tar, although seal-coating can also be caused by otherconditions that are described below. Seal-coating may occur when sootand/or tar deposit excess carbon early in the densification process onsurfaces of the stack of porous structures. As the carbon accumulates onthe surfaces of the stack of porous structures, the surface poreseventually become blocked (i.e., occluded), or sealed, thus preventingthe flow of reactant gas from further permeating the stack of porousstructures. As a result, densification of the interior region around theseal-coated surface prematurely stops, thereby potentially leavinginterior porous defects in the finished carbon part (i.e., the densifiedpreform).

To address the occlusion of pores of a stack of porous structures,conventionally, multiple densification steps were employed. Statedanother way, a CVI/CVD process would be stopped, the furnace allowed tocool, and the stack of porous structures would be extracted and machinedto open the pores. Then, the stack of porous structures would be placedinto the furnace and the CVI/CVD process would commence again in asecond CVI/CVD process step. The rearrangement and machining of thestack of porous structures between cycles (steps) is costly andtime-consuming. Thus, in various embodiments, disclosed herein is aCVI/CVD process that may begin with a stack of porous structures thathas previously not undergone a CVI/CVD process and achieve acommercially viable density of that stack of porous structures (e.g.,reaches a density in the range from 1.6 g/cc to 1.9 g/cc), and invarious embodiments, a density of approximately 1.75 g/cc) in a singlecycle. In this regard, in various embodiments, stack of porousstructures may be manufactured without use of multiple cycles.

In various embodiments, CVI/CVD processes are disclosed herein, wherein-process modifications to a flow of reactant gas and/or exhaust gasare implemented in a single processing cycle. Modifying reactant gasflow direction reconfigures a pressure differential direction (e.g.,switching high-pressure and low-pressure sides) during a CVI/CVD processcycle, thereby allowing a commercially-viable density to be achieved ina single processing cycle.

A number of different types of furnaces may be used for CVI/CVDprocesses. Typically, a furnace includes a cabinet that encloses agraphite susceptor. The graphite susceptor encloses one or more stacksof porous structures that are to undergo a CVI/CVD process.

In various embodiments, and with reference to FIG. 1, CVI/CVD apparatus100 is illustrated. Spacers 86 are disposed between the porousstructures 110, which may comprise one or more carbon fibers,effectively dividing the space within the graphite susceptor to createan inner diameter (ID) volume 190 and an outer diameter (OD) volume 185and sealing an inner diameter (ID) volume 190 from OD volume 185.Spacers 86 may comprise solid rings. Spacers 86 may also comprise ringswith voids configured to allow fluid communication between OD volume 185and ID volume 190, in various embodiments. Where a solid ring is usedfor spacers 86, the pressure differential may develop more rapidlybetween OD volume 185 and ID volume 190 than where spacers 86 comprise arings having voids. In various embodiments, spacers 86 comprise bothsolid rings and rings having voids. Spacers 86 may comprisecarbon/carbon, graphite, and/or any other suitable material. Spacers 86may also comprise a surface coating to prevent spacers 86 from adheringto porous structures 110. Porous structures 110 are annular and in thatregard porous structures 110 comprise one or more annular porousstructures. The ID volume 190 and OD volume 185 are not in fluidcommunication, except by way of the pores in the porous structures 110.The ID volume 190 is shown radially inward from an ID surface 193 on aninner diameter of porous structures 110. Therefore, ID volume 190 may bedefined by the inside surface diameter of porous structures 110 andspacers 86. The OD volume 185 is shown radially outward from OD surface192 of porous structures 110. Therefore, the OD volume 185 may bedefined by the outside diameter of porous structures 110, spacers 86,and the inside surface of the graphite susceptor 102. The OD volume isshown radially outward from OD surface 192 of porous structures 110.

In various embodiments, the graphite susceptor 102 may be disposedwithin furnace 105 and may be induction heated by an induction coil orgas flame. Although induction heating is described herein, other methodsof heating may also be used such as gas heating, resistance heating andmicrowave heating, any of which are considered to fall within thepresent disclosure.

To provide for the flow of reactant gas and to facilitate discharge ofreactant gas exhaust, the furnace includes a number of inlets andoutlets. Gas inlets 103, 52 and 50 are configured to allow reactant gasto flow into furnace 105. Outlet 54 is configured to allow reactant gasto flow out furnace 105. Valve 140 is configured to allow reactant gasto flow through gas inlet 103. Valve 145 is configured to allow reactantgas to flow through gas inlet 50. Valve 135 is configured to allowreactant gas to flow through gas inlet 52. Valve 155 is configured toallow reactant gas to flow out outlet 54. Vacuum pump 122 is in fluidcommunication with valve 155 and provides a suction source to evacuatefurnace 105. Reactant gas source 115 may be in fluid communication withvalve 88. Thus, actuation of valve 88 may operate to supply reactant gasto valves 135, 140, and 145.

In various embodiments, a preheater is included to heat the reactant gasbefore the reactant gas flows into ID volume 190 or OD volume 185. Forexample, a preheater may comprise a series of graphite plates withvoids, heated by an induction coil and/or graphite susceptor 102. As thereactant gas passes through the preheater, it may be heated to a definedtemperature. Typically, preheaters are sealed, such that an incomingreactant gas flowing from a gas inlet (e.g., gas inlets 52, 103, and 50)is received by the preheater and heated to the defined temperature. Theheated reactant gas then enters graphite susceptor 102 by way of one ormore openings (e.g., openings 125, 126, and 127) in the base of graphitesusceptor 102. For example, preheaters 60, 4, and 58 may preheatreactant gas prior to entry of the reactant gas into ID volume 190 or ODvolume 185.

Graphite susceptor 102 includes openings 125, 127 which open into ODvolume 185. Graphite susceptor 102 includes opening 126, which opensinto ID volume 190. Graphite lid 175 of graphite susceptor 102 includesthree openings 170, 171, and 172. Plug 62 fills opening 171, and,referring to FIG. 2, plug 64 fills opening 170 and plug 66 fills opening172. Plugs, such as plug 62, may comprise graphite or other suitablehigh temperature material. In various embodiments, plug 62 is a solidgraphite weight that creates and maintains an airtight seal of a lid byforce of gravity asserting significant downward pressure over opening171. In various embodiments, the plug 62 is positioned above stack ofporous structures 110, such that the plug 62 is wedged between a stackof porous structures 110 and an inner surface of graphite lid 175.

In various embodiments, a pressure differential may be created byfeeding reactant gas into one volume, such as ID volume 190. Because IDvolume 190 and OD volume 185 are fluidly connected only through porousstructures 110, a pressure differential will be formed in the volumewhere reactant gas is fed, which in turn forces reactant gas throughporous structures 110. To withstand a pressure differential, plugs 62,64, and 66 may be used to seal graphite lid 175 and contain the reactantgas under pressure. In various embodiments, the seal between graphitelid 175 and porous structures 110 may ensure the generation of apressure differential between ID volume 190 and OD volume 185.

In various embodiments, furnace 105 may be configured for an innerdiameter (ID) feed. Reactant gas flows into the ID volume 190 by way ofvalve 140 and through opening 126. Plug 62 prevents reactant gas fromescaping the ID volume 190. Increasing pressure within ID volume 190pushes the reactant gas through the one or more pores of porousstructures 110. In this regard, the pressure within ID volume 190builds, to pressures of, for example, between 1 mmHg (133 Pa)-75 mmHg(9999 Pa), between 20 mmHg (2666 Pa)-50 mmHg (6666 Pa), and between 35mmHg (4666 Pa) and 45 mmHg (5999 Pa). A portion of the reactant gasdecomposes or pyrolyzes and is deposited within porous structures 110.Reactant gas flows in the direction of the outer diameter (OD) volume185 by way of porous structures 110. In various embodiments, thereactant gas is drawn out the OD volume 185 through openings 170 and 172and out the furnace 105 through outlet 54. Vacuum pump 122 providessuction through valve 155.

During a CVI/CVD process, the flow of reactant gas may be altered tochange the pressure differential between the ID volume 190 and OD volume185. For example, with reference to FIG. 2, mechanical controller 76 maycontrol arm 78 to remove plug 62 from opening 171. Control arm 78 mayplace plug 64 in opening 170 and plug 66 in opening 172. In variousembodiments, mechanical controller 76 is remotely controlled by anoperator. In a remotely controlled configuration, mechanical controller76 replicates or interprets the movements of an operator, who ispositioned remotely at a safe distance from furnace 105. Control arm 78may be controlled by way of instructions from one or moremicrocontrollers/microprocessors in mechanical controller 76. Sensorsmay be configured to provide information that may be used by themicrocontroller/microprocessor to formulate instructions for controllingmechanical implement movements, for example.

In that regard, OD volume 185 may be sealed and ID volume 190 may becomeopen through opening 171. Reactant gas may be fed through valves 135 and145 into openings 125 and 127, thereby entering OD volume 185. Pressuremay then build in OD volume 185, forcing the reactant gas to flowthrough OD surface 192 of porous structures 110. Reactant gas travelsthrough the porous structures 110 and exits through ID surface 193, thusentering ID volume 190. After the reactant gas enters ID volume 190, itis expelled from ID volume 190 through an opening 171 in graphite lid175. Vacuum pump 122 provides suction, through valve 155, which drawsthe reactant gas exhaust out furnace volume 165.

To alter a pressure differential, the reactant gas feed may be toggledbetween ID volume 190 and OD volume 185. When reactant gas is to be fedinto OD volume 185, valves 135 and 145 may be opened, and valve 140 maybe closed, allowing reactant gas to travel into OD volume 185. Valve 155may then be actuated to exhaust the reactant gas. When reactant gas isto be fed into ID volume 190, valve 140 may be opened, and valve 135 and145 may be closed, allowing reactant gas to travel into ID volume 190.Valve 155 may then be actuated to exhaust the reactant gas.

Reconfiguring the pressure differential and gas flow direction (i.e.,changing which volume reactant gas enters and leaves) within graphitesusceptor 102 may take place one or more times in the course of aCVI/CVD process, unlike in a typical CVI/CVD process where only one sideof the stack of porous structures 110 is exposed to the higher side of apressure differential. Reversal of the gas flow direction and pressuredifferential within a CVI/CVD process may offer potential advantages ofa higher densification rate and increased density uniformity of thefinal carbon composite part.

Additional factors influence deposition uniformity including, forexample, CVI/CVD process temperatures, graphite susceptor 102 volumepressures, reactant gas concentrations, geometry of furnace 105,geometry of graphite susceptor 102, HOG, etc. In various embodiments,changing the reactant feed between ID volume 190 and OD volume 185 or ODvolume 185 and ID volume 190 may be performed once in a single cycle.Changing the flow of reactant gas (e.g., changing the feed from IDvolume 190 to OD volume 185) may occur once during a CVI/CVD process,for example, after between about 50 to about 400 HOG, from about 100 toabout 300 HOG, and from about 200 HOG to 250 HOG, where the term aboutin this context only means+/−10 HOG.

In various embodiments, the above reconfiguring/switching processes mayalso be implemented through inclusion of reversing valves within thefurnace hardware configuration. This approach may provide a costeffective option that may also allow for retrofitting existing furnacesto achieve similar disclosed functionality.

This disclosure describes various embodiments relating to CVI/CVDprocesses as well as discussions relating to various hardware componentsand hardware configurations for carrying out the processes. One suchhardware component is a valve, which is described herein as having avariety of functions relating to variously disclosed implementations,features, and functions. However, the inventive features disclosedherein are not solely reliant on the presence or lack thereof of one ormore valves.

Referring to FIG. 3, CVI/CVD apparatus 300 is shown. Graphite susceptor302 may include a number of inlets/outlets (i.e., openings) for allowinggasses to flow into and out the graphite susceptor 302. Removable plugsmay be used to configure the graphite susceptor 302. For example, plugsmay be fitted into each of openings 377, 325, and 327. Control overgraphite susceptor 302's configuration may be provided by such plugs,which allow the processes disclosed herein to be implemented through useof existing furnace configurations with minimal modification orretrofitting.

Line 195 may be attached to gas inlet/outlet 360. Line 195 may thusplace reversing valve 402 into fluid communication with furnace 305.Line 194 may be attached to gas inlet/outlet 362. Line 194 may thusplace reversing valve 402 into fluid communication with furnace 305. Invarious embodiments, line 195 and line 194 may comprise a pipe, conduit,or other suitable device for conveying fluids.

With reference to FIG. 3 and FIG. 4, reversing valve 402 comprises fourports, two ports designated to function as gas inputs and two portsdesignated to function as gas outputs. Reversing valve 402 may exist intwo states: energized or unenergized. Based on the state of thereversing valve 402 (e.g., energized or unenergized), the designatedfunctions of two of the four ports may be reversed. Stated another way,in the energized state, two ports act as inlets and two act as outletsand in the unenergized state, one of the ports that act as an inlet inthe energized state acts as an outlet and one of the ports that acts asan outlet in the energized state acts as an inlet. In particular, theflow in lines 195 and 194 may be reversed in response toenergizing/unenergizing. Reversing valve 402 also includes an electronicactuation system to switch reactant gas flow direction in response to aninstruction from at least one of an operator or a microcontroller (e.g.,a processor). Reversing valve 402 reconfigures port designations inresponse to being switched to an energized state or an unenergizedstate, each state corresponding to a state of an electronic actuationsystem state of reversing valve 402.

In various embodiments, methods and configurations are described forconfiguring a reactant gas pressure differential among OD volume 385 andID volume 390 during CVI/CVD processing. In various embodimentsdescribed herein, toggling reactant gas feed between ID volume 390 andOD volume 385 can be achieved by periodically energizing andun-energizing a reversing valve 402 in accordance with variousembodiments.

The graphite susceptor 302's opening for reactant gas feed and theopenings for reactant gas exhaust may be reversed, thereby allowing oneto change the direction of the flow of reactant gas and switch thepressure differential with respect to the stack of porous structures310. Spacers 386 are disposed within porous structures 310. Thefunctions of the openings (i.e., inputs, outputs) are defined by thestate of reversing valve 402. The reversing valve 402 may comprise afirst port 404 and a second port 406, wherein the first port 404 iscoupled to opening 326 in the graphite susceptor 302 via line 194. Thesecond port 406 is coupled to second openings 370 and third opening 372in graphite susceptor 302 via line 195. Plug 399 is disposed in opening377, plug 350 is disposed in opening 325 and plug 354 is disposed inopening 327.

With reference to FIG. 3, in accordance with various embodiments, gasenters ID volume 390 by way of opening 326 in response to energizingreversing valve 402, which allows reactant gas to flow from line 194that transports reactant gas from source 292 to be fed through opening326. Thus, reversing valve 402 is in the energized state in FIG. 3. Invarious embodiments, the reactant gas flows into ID volume 390 andcontinues as pressure builds in ID volume 390. The pressure in ID volume390 forces the reactant gas to flow through ID surface 393 of porousstructures 310 and through OD surface 392 of porous structures 310.Reactant gas thus enters OD volume 385. Reactant gas may then be drawnthrough openings 370 and 372 and through inlet/outlet 360 into line 195.Reactant gas may then be drawn through reversing valve 402 towards avacuum pump 196.

The reversing valve 402 may then unenergized, with reference to FIG. 4.In accordance with various embodiments, gas enters OD volume 385 by wayof openings 370 and 372 in response to unenergizing reversing valve 402,which allows reactant gas to flow from line 195 that transports reactantgas from source 292 to be fed to openings 370 and 372 throughinlet/outlet 360. In various embodiments, the reactant gas flows into ODvolume 385 and continues as pressure builds in OD volume 385. Thepressure in OD volume 385 forces the reactant gas to flow through ODsurface 392 of porous structures 310 and through ID surface 393 ofporous structures 310. Reactant gas thus enters ID volume 390. Reactantgas may then be drawn through opening 326 and through inlet/outlet 362into line 194. Reactant gas may then be drawn through reversing valve402 towards a vacuum pump 196.

With reference to FIGS. 1, 2 and 5, method 500 is illustrated. Achemical vapor infiltration process for an inner diameter (ID) reactantgas feed is illustrated. One may open a first valve to start reactantgas to flow into ID volume 190 (Step 505). Reactant gas flows through IDsurface 193 of the porous structures 110 (Step 510). Reactant gas flowsinto OD volume 185 (Step 525). After one or more HOG, the reactant gasflow may be altered. Plug 62 may be removed while plugs 64 and 66 may beset into place (Step 528). Reactant gas flows into the OD volume 185(Step 530). As the reactant gas flows through porous structures 110, itflows into the ID volume 190 (Step 535). The reactant gas is thenreleased from ID volume 190 as exhaust by way of opening 171 (Step 540).

Referring to FIGS. 3, 4 and 6, method 600 is illustrated. A reversingvalve may be energized (Step 605), causing line 194 to function as gasinput line and line 195 to function as an exhaust line. Reactant gasenters the ID volume by way of opening 326 and flows through ID surface393 (Step 610). The reactant gas then enters the OD volume 385 (Step620). The reactant gas is released from OD volume by way of openings370, 372 (Step 625). Reversing valve is then un-energized, causing line195 to function as gas input line and line 194 to function as exhaustline (step 630). Reactant gas enters OD volume 385 by way of openings370, 372, increasing the pressure in OD volume 385 (Step 635). Thereactant gas flows from OD volume through the porous structures 310 inthe direction of the ID volume 390 (Step 640). The reactant gas entersID volume 390 through porous structures 310 (Step 645) and is releasedby way of opening 326 (Step 650).

During a CVI/CVD process, a furnace may typically operate attemperatures around 1025° C. (1877° F.), while heat treatment processesperformed after the CVI/CVD process in the same furnace may operate ashigh as 2000° C. (3632° F.). As such, materials used in the manufactureof various furnace components shown in FIGS. 1 to 4, may be selectedbased on a material's ability to withstand extremely high temperatures.Alternatively, furnace components may be positioned away from areas inand around the furnace where the temperatures are highest.

Benefits, other advantages, and solutions to problems have beendescribed herein with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any elements that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as critical, required, or essentialfeatures or elements of the invention. The scope of the invention isaccordingly to be limited by nothing other than the appended claims, inwhich reference to an element in the singular is not intended to mean“one and only one” unless explicitly so stated, but rather “one ormore.” Moreover, where a phrase similar to “at least one of A, B, or C”is used in the claims, it is intended that the phrase be interpreted tomean that A alone may be present in an embodiment, B alone may bepresent in an embodiment, C alone may be present in an embodiment, orthat any combination of the elements A, B and C may be present in asingle embodiment; for example, A and B, A and C, B and C, or A and Band C. Furthermore, no element, component, or method step in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element, component, or method step is explicitly recited inthe claims. No claim element herein is to be construed under theprovisions of 35 U.S.C. 112, sixth paragraph, unless the element isexpressly recited using the phrase “means for.” As used herein, theterms “comprises”, “comprising”, or any other variation thereof, areintended to cover a non-exclusive inclusion, such that a process,method, article, or apparatus that comprises a list of elements does notinclude only those elements but may include other elements not expresslylisted or inherent to such process, method, article, or apparatus.

1. An apparatus for densifying an annular porous structure comprising: agraphite susceptor supporting the annular porous structure, wherein theannular porous structure defines an inner diameter (ID) volume and anouter diameter volume (OD), wherein the graphite susceptor comprises alid; a first plug disposed in a first opening of the lid, the firstopening in fluid communication with the ID volume; a second plugdisposed in a second opening of the lid, the second opening in fluidcommunication with the OD volume; a third plug disposed in a thirdopening of the lid, the third opening in fluid communication with the ODvolume; and a control arm configured to remove at least one of the firstplug, the second plug, or the third plug.
 2. The apparatus of claim 1,wherein the graphite susceptor is disposed in a furnace.
 3. Theapparatus of claim 2, wherein the furnace is at least one of gas heatedor induction heated.
 4. The apparatus of claim 2, wherein the controlarm is configured to insert the first plug into the first opening, thesecond plug into the second opening, and the third plug into the thirdopening.
 5. The apparatus of claim 1, wherein the control arm iscontrolled by a mechanical controller.
 6. The apparatus of claim 2,further comprising a reactant gas source is in fluid communication withthe furnace and configured to provide a reactant gas, wherein thereactant gas comprises at least one of methane, ethane, propane,cyclopentane, hydrogen, nitrogen, helium, argon, or an alkane.
 7. Theapparatus of claim 1, wherein the annular porous structure comprises acarbon fiber.